An emulsion polymerized PTFE is obtained by polymerizing the tetrafluoroethylene (hereinafter referred to as TFE) monomer, in the presence of water, a polymerization initiator, an anionic perfluorocarboxylate surfactant (hereinafter referred to as APFC) and a paraffin wax stabilizer, and it is obtained as an aqueous PTFE polymer emulsion containing PTFE particles.
The aqueous PTFE polymer emulsion after the polymerization is used directly, after concentration, or as an aqueous PTFE dispersion composition after blended with a surfactant, a filler or other known components as the case requires. However, it has not been industrially common to specially remove APFC from an aqueous PTFE dispersion composition.
After used in polymerization of PTFE, expensive APFC is preferably recovered as much as possible for reuse.
In order to remove APFC from the aqueous PTFE dispersion composition, WO 00/35971 proposes adsorption of APFC by passing an aqueous PTFE dispersion composition through an ion exchange resin. However, there is a problem that adsorption performance deteriorates since PTFE particles occlude the ion exchange resin.
In order to remove APFC from the aqueous PTFE dispersion composition, WO00/35971 proposes adsorption of APFC by passing an aqueous PTFE dispersion composition through an ion exchange resin. However, there is a problem that adsorption performance deteriorates since PTFE particles occlude the ion exchange resin.
Further, WO 01/79332 has proposed volatilization of APFC by boiling an aqueous PTFE dispersion composition. However, there is a problem that agglomeration during the treatment tends to lower the yield.
It is an object of the present invention to provide a method for lowering the APFC concentration in the aqueous PTFE dispersion composition efficiently and an aqueous PTFE dispersion composition obtained by the method.
The present inventors have conducted extensive studies to overcome the above problems and as a result, have discovered that APFC adsorbed on the surface of the PTFE particles efficiently transfers to the aqueous phase upon addition of a specific amount of a specific nonionic surfactant and water to the aqueous PTFE polymer emulsion, and have found that the APFC concentration in the aqueous PTFE dispersion composition can be decreased drastically by removing the aqueous phase in a concentration step which comprises sedimenting PTFE and separating a PTFE-rich aqueous dispersion composition from the supernatant. The present invention has been accomplished on the basis of these discoveries. It is possible to recover a large amount of APFC from the aqueous phase.